In a known power steering system, an engine driven pump provides a fixed volume of fluid output per revolution of the pump impeller during operation of the pump. Therefore, the rate of flow of fluid from the engine driven pump is proportional to engine speed. The pump in this known power steering system is sized to provide an acceptable rate of fluid flow when the engine is idling.
A known power steering motor assembly has an open center valve, which requires a constant rate of fluid flow independent of engine speed. In order to obtain a constant rate of fluid flow independent of engine speed, a known power steering system utilizes a flow control valve having a spring loaded valve spool, which is moved to maintain a constant pressure drop across a control orifice.